Passenger flow statistics based on flow directions of persons is very important for commercial places, such as shopping malls, supermarkets and the like, to understand and adjust their business strategies. At present, a method for detecting a flow direction of a person usually sets at the entrance region of a commercial place a single image acquisition apparatus and presets a straight line therein, and then judges the flow direction of the person according to whether and in which direction the person passes through the straight line. However, in accordance with such detection method, when the movement trajectory of the person to be detected (i.e. the target object) is not regular, it is very easy to yield false negatives or false positives. On the other hand, when a commercial place is an irregular site, it may not be able to simply decide whether a person to be detected enters the commercial place or not by setting a straight line in a single image acquisition apparatus. For example, it is assumed that a certain commercial place is located on both the left and right sides of a crossroads, then turning left and turning right at the crossroads may both lead to entering the commercial place. At this point, presetting a straight line in a single image acquisition apparatus fails to detect all persons entering the commercial place.